Water-based dye ink containing a black dye has heretofore mainly been used as a recording solution for a writing implement or an ink-jet printer. However, in order to impart light and water resistances to a recorded image, water-based pigment ink which contains a pigment such as carbon black has been desired.
Carbon black commercially available for a colorant is acidic carbon black having a pit of not more than 7, which is prepared by oxidizing neutral carbon black with an oxidizing agent such as nitric acid. The neutral carbon black generally means a carbon black having a pH of about 7 prepared by cracking or imperfectly burning natural gas, and a hydrocarbon liquid such as heavy oil, tar and the like. The acidic carbon black itself does not have enough water-distensibility for use as a colorant of water-based pigment ink.
Therefore, carbon black is conventionally finely dispersed and stabilized in an aqueous medium by using various dispersing apparatuses in the presence of a dispersant such as water-soluble acrylic resin, styrene(meth)acrylic resin, styrene-maleic resin and amine.
Japanese Laid-Open Publication Nos. 64-6074 and 64-31881, for example, disclose water-based pigment ink which comprises acidic carbon black having a pH of not more than 5 and a dispersant (e.g., an anionic surfactant and a polymeric dispersant). Japanese Laid-Open Publication No. 3-210373 discloses ink for ink-jet printing which comprises acidic carbon black and a water-soluble resin.
In order to generate liquid drops stably through a fine orifice of an ink-jet printing head and to write smoothly with a fine tip of a ball point pen, the orifice or the tip have to be prevented from drying and setting.
When the water-based pigment ink comprising a dispersant is employed for ink-jet printing or fine-pen writing, a resin of the dispersant adheres to the orifice or the tip. The adhered resin is difficult to dissolve again, and the orifice or the tip may clog easily. Further, the water-based ink containing a dispersant is viscous, and it forms resistant force in a path to a nozzle of the ink-jet printing head when continuous or high-speed printing is conducted for a long period of time. The resistant force makes jetting of the ink unstable, which results in intermittent printing and recording.
Furthermore, such a conventional water-based pigment ink has to have relatively low concentration in order to reduce viscosity of the ink, which results in poor printing density as compared with water-based dye ink.